1. Field of the Invention
This invention relates to methods of thinning a workpiece, and more particularly, to methods of making diode-array targets from semiconductive substrates for electron-beam charge-storage devices such as television camera tubes.
2. Description of the Prior Art
This invention is particularly suited for use in the manufacture of semiconductive devices or the like. An example of such semiconductive devices are silicon targets having arrays of light-sensitive photo diodes for the storage of electron-beam charges, such as those disclosed in Reynolds U.S. Pat. No. 3,011,089 and Buck et al. U.S. Pat. Nos. 3,403,284 and 3,458,782.
While the invention is adapted to be used in conjunction with the masking of a number of different kinds of workpieces, it will be particularly described with respect to masking semiconductive substrates. These substrates are advantageously used in making diode-array targets for electron-beam charge-storage devices such as television camera tubes. The diode-array target is basically a flat semiconductive substrate having a closely spaced array of p-n junctions near one surface.
In fabricating the diode-array target in accordance with prior art techniques, an n-type substrate is prepared by sawing it from a silicon crystal ingot and then etching and polishing it. After a careful cleaning, a layer of silicon dioxide is formed over the entire substrate.
Next, a photoresist material is deposited on the face of the substrate and a fluid is directed on its backside. The substrate is then spun to coat evenly the face, while the fluid prevents the photoresist material from coating the backside. (See, e.g., co-pending application, Ser. No. 95,821, filed on Dec. 7, 1970 by L. F. Boyer and A. F. Johnson, Jr., assignors to Western Electric Co., Inc. and now issued into U.S. Pat. No. 3,695,928) The substrate is then exposed with the required diode-array pattern and developed to form apertures in the photoresist material. Etching of the substrate through these apertures follows to produce corresponding apertures in the silicon dioxide layer. Through the apertures in the silicon dioxide layer, boron is diffused to form p-type regions, with the dioxide layer acting as a diffusion mask. These p-type regions in the n-type substrate form the diode array on one side of the substrate.
Following the formation of the diode array, the substrate is subjected to a thinning operation involving numerous process steps. First, wax (such as that sold under the trademark "Apiezon") is applied to a sapphire supporting disc heated by a hot plate. In applying the wax an opening is left in the center of the disc for subsequent optical measurements for ascertaining the progression of the thinning of the substrate. The substrate is then placed on top of the waxed disc with the array side facing down and carefully centered. The assembly of the disc, wax and substrate is then removed from the hot plate and allowed to cool, leaving the substrate held to the disc by the wax.
Using a rotating tool, a rim of wax in a solvent is applied to the backside of the substrate using a small brush. After the wax dries, the central portion of the backside of the substrate is subjected to an etching operation wherein the substrate is immersed in an etchant and rotated therein for a period sufficient to thin the substrate down to a predetermined thickness. This thickness is considerably less than the diffusion length of minority carriers generated by absorbed light in the ultimate target and limits the amount of lateral diffusion of minority carriers in order to obtain high resolution in the target.
After the thinning, the substrate is rinsed, blown dry and optically measured. The substrate is then removed from the supporting disc by placing the assembly on the hot plate to soften the wax. The slice is mechanically slid off the sapphire disc and placed in three successive hot trichloroethylene baths and then in a boiling trichloroethylene solution followed by an alcohol soak and water rinse. This results in the removal of all traces of wax.
After the removal of the substrate from the disc, it is subjected to several finishing heat treatments. The first of these treatments is a shallow phosphorous diffusion to improve the blue sensitivity of the ultimate camera tube and to reduce its dark current. Next, the boron diffusion glass, which has been left on the array side of the substrate up to this point to protect it against phosphorous diffusion, is removed to expose the p-type regions of the diodes. At the same time that the boron diffusion glass is removed, the phosphorous diffusion glass is also removed.
Next, the substrate is annealed in hydrogen at a low temperature to further reduce the dark current of the ultimate camera tube. Finally, a resistive film is evaporated over the diode array and it is ready for evaluation. See Crowell et al U.S. Pat. No. 3,419,746.
The holding of the substrate in place with the wax and the other processing steps necessitated by the wax results in a messy, time-consuming process involving numerous process steps. These steps do not lend themselves to high-volume production. In addition, there are a number of impurities found in the wax, such as iron, magnesium and calcium. These impurities often lead to the contamination of the ultimately formed diode-array target.
In fabricating the diode array target, it is desirable to eliminate as many process steps as possible, especially the wax step. One prior art technique of eliminating the waxing step is to use a mechanical mask held to the slice by magnetic facilities. However, difficulty was experienced with this as a result of the etchant leaking to the array side of the substrate and rendering defective the ultimately fabricated target.
It is also desirable in the thinning operation, which results in the removal of about 90% of the material at the central portion of the substrate, that the removal take place in a controlled and uniform manner. This is because irregular etching results in high spots, holes, depressions and other surface non-uniformities, which render defective the ultimately fabricated diode-array target.